Various communication systems are known for transmitting through the atmosphere. Most commonly, microwave communication devices are used for this communication. Additionally, various optical techniques for communication are known.
Microwave technologies used for data links through the atmosphere generally suffer from low data rates. For example, an 11 to 14 or 20 to 30 gigahertz (GHz) carrier is sometimes used to transmit between a satellite and a ground station. However, modulating 1 gigabit/sec. data rates on the 18 GHz carrier is not currently practically accomplished. By increasing the carrier frequency, the data rate may also be increased. For example, a 60 GHz carrier could support modulation data rates of 1 gigabit/sec. or higher. However, the atmosphere itself attenuates the 60 GHz carrier such that transmission through the atmosphere is not practical.
Terrestrial-based free space optical transmission systems are being considered as a replacement for free space microwave communication systems. Laser communication systems theoretically can transmit through the atmosphere at high data rates. Laser communication systems must be properly designed to minimize the risk of high bit error rates (BER) due to atmospheric disturbances from thermal air currents and inclement weather conditions.
The effects of atmospheric disturbances from thermal air currents along the optical path are typically greatest at the transmitter and receiver. For a terrestrial link, the transmitter and receiver are typically (compared to other parts of the optical path) in closest proximity to thermal sources, namely solid surfaces such as rooftops, which absorb solar flux and warm the air. The warm air rises through cooler air thereby generating turbulence and providing a non-uniform propagation medium for the optical signal. At the transmitter, such disturbances can cause different portions of the optical beam to pass through differently moving air currents, causing the propagating beam to break up, so leaving the receive aperture in an intensity minimum. Similar disturbances near the receive aperture can cause the focal spot to break up, leaving the detector in an intensity minimum. Each of these distinct processes can produce burst errors in the communicated data and must be addressed separately.
Space-based optical communication links between widely separated laser transceivers are generally designed to minimize the pointing criticality, aperture diameters for a given link range, laser power and data rate, by use of matched, common transmitter and receiver apertures. This symmetrical link geometry serves, in a homogeneous medium such as free space, to provide the maximum power transfer for the smallest terminal mass and volume. When the cost of dichroically separating the transmit and receive paths within the transceiver is prohibitive, a smaller transmit aperture is commonly located near a large receive aperture at each transceiver. Such an asymmetrical transceiver design is particularly common for atmospheric laser links to minimize the detrimental effect of atmospheric disturbances on BER. Even though a smaller transmit aperture can reduce interference, beams can still wander off or destructively interfere at the intended receiver. To address this problem, multiple (typically four) redundant transmitters are often used to reduce the probability of deep fades in the received power. This arrangement can still be inadequate, particularly in the afternoon when solar-induced thermal currents are most severe.